Warm-blooded giants of the ancient seas?

Isolated tooth of an elasmosaurid plesiosaur from the late Cretaceous of Morocco.
Image courtesy of Christophe Lecuyer

Some of the giant reptiles that ruled the ocean food chain during the time of the dinosaurs may have been able to control their own body temperatures, a new study suggests. These reptiles probably had high metabolic rates, which helped them dive deep and swim fast over large distances to catch their prey.

Three large, extinct swimming reptiles, the ichthyosaurs, plesiosaurs and mosasaurs, were the top ocean predators during the Mesozoic era, about 251 to 65 million years ago.

Most reptiles today are cold-blooded, meaning their body temperature is determined by how warm or cold their surroundings are. But, some of the modern ocean's top predators, tuna and swordfish, are "homeothermic," or able to keep their body temperatures at a constant temperature despite changing environmental conditions.

To see whether the three lineages of Mesozoic marine reptiles were also homeothermic, Aurélien Bernard of CNRS and Université Lyon in France and colleagues analyzed various types of oxygen in the teeth of these reptiles. They compared the oxygen in the reptile teeth with the oxygen in the teeth of fish from the same environments.

Isolated tooth of the ichthyosaur Platypterigius from the late early Cretaceous of England.
Image courtesy of Christophe Lecuyer

This tooth oxygen is a clue to an animal's body temperature, because it reflects the composition of oxygen in the blood.

The researchers knew that the fish whose teeth they were studying were cold-blooded. So, when they found reptile teeth with different oxygen signatures, it probably meant that those reptiles had warmer body temperatures than the fish did.

The results suggested that ichthyosaurs and plesiosaurs, which chased their prey, probably controlled their own temperatures. The data for mesosaurs, which are thought to have hunted by ambush, were less clear, but it's possible that these reptiles could control their body temperature to some degree.